Switching networks are used for connections between terminals in telecommunication switching systems. Two types of switching connections are distinguished, i.e. the space and the time division type. Space division uses for this purpose switching elements which are operated temporarily, whereas a periodically recurring pulsephase or time slot is assigned to a time-division connection.
Switching networks for pulse code modulation (PCM) using both time and space division switching are state of the art. The number of the switching stages depends on the size of the switching network, i.e. on the amount of traffic to be handled and the design of the switching network.
There are two types of trunking schemes, the elongated trunking scheme with terminals on both sides, and the reverse trunking scheme with all terminals on one side of the switching network. Switching networks of the latter type are distinguished by a nearly perfect availability of the outgoing lines. Such a switching network is disclosed for example in Reports on Telephone Engineering VI (1970) 1/2 published by Siemens AG, Berlin and Munich, West Germany, pp. 28 to 33.
If as a first stage in a PCM time-division multiplex switching network the time-division type is used, the number of terminals to be connected to a transmitting and a receiving highway is limited because of transmission and teletraffic reasons. If additional terminals are to be connected to the switching network, the first state is divided up into several switching groups, whereby connections between terminals of different switching groups are switched via a second stage. Since the capability of the second switching stage is also limited, it is useful to divide up the switching groups into major switching groups, whereby connections between the major switching groups are switched via a third switching stage, etc.
A multistage PCM switching network with space division and time division switching for use as a transit exchange is disclosed in the German patent application Ser. No. AS 24 15 367 either as a switching network of the elongated (FIG. 2) or of the reversed (FIG. 1) trunking scheme. By the use of combined space division and time division switching stages better traffic conditions can be attained, but the technical equipment per PCM-line is relatively high because of the individual information stores. Furthermore a multistage switching network consisting merely of time division switching stages with a reversed trunking scheme is disclosed in each of the German patent application Nos. AS 24 54 090 and OS 24 54 144, whereby the latter patent application is identical with the U.S. Pat. No. 4,025,725 of May 24, 1977.
It is an object of the invention to provide a PCM switching network consisting of time and space division switching stages, whereby the individual switching stages are arranged in such a manner that their qualities are used optimally, although the technical equipment remains small.